1. Field of the Invention
This invention relates to a method of manufacturing plasma display panels, referred to hereinafter as PDPs, in which a pair of substrates with a discharge space therebetween is vacuum sealed along the respective peripheries thereof, and particularly relates to a sealing method to form such a panel having such a sealed discharge space.
2. Description of the Related Art
Hereinafter is described a structure of an AC-driven three-electrode surface discharge type PDP, as representative of plasma display panels in which the present invention can be embodied. As shown in FIG. 19, a perspective and partially cross-sectional view of a PDP, there is arranged for each line L of a display matrix a pair of display electrodes X and Y upon an inner surface of a front glass substrate 50 in order to generate a surface discharge along a surface of the front substrate 50. The display electrodes X and Y may also be called sustain electrodes. The display electrodes X and Y are respectively formed of a stack, or laminate, of a wide, straight transparent electrode 52 formed of a thin film of ITO, Indium Tin Oxide, and a narrow straight bus electrode 53 formed of a thin metal film. The display electrodes X and Y are formed by means of a photolithography technique.
A dielectric layer 54 for the AC (alternating current) drive is formed on the inner surface of the front substrate 50, so as to cover the display electrodes X and Y and protect same from discharges in the discharge space, by means of a screen printing method. Upon dielectric layer 54 is deposited a protecting layer 55 formed of MgO, Magnesium Oxide.
On the other hand, upon an inner surface of a back glass substrate 51 there are arranged, in order to generate address discharges, address electrodes 56, orthogonal to the display electrodes X and Y and spaced by a constant pitch. The address electrodes 56 as well preferably are formed of a stack, or laminate, of metal films by means of a photolithography technique.
Upon the entire inner surface of the back glass substrate 51, including the portions above the address electrodes 56, there is formed a dielectric layer 57 by means of a screen printing method and, further, thereupon is provided a plurality of approximately 150 xcexcm high straight separator walls, or barriers, 58 each centered between a respective pair of adjacent address electrodes 56. Fluorescent materials 60, of three primary colors R (red), G (green) and B (blue) for a full color display, are coated so as to cover the surface of dielectric layer 57 including the respective, exposed portions above corresponding address electrodes 56 and the sides of the separator walls 58, by means of a screen printing method.
Within discharge space 59 is filled a discharge gas, such as typically a mixture of Nexe2x80x94Xe, i.e. neon gas and xenon gas, of several hundreds Torr, for exciting the fluorescent materials by irradiating thereon ultra-violet rays during the gaseous discharge. A sealant (seal-glass layer) 61 is provided for sealing the discharge space 59 at the respective peripheral portions of the substrates 50 and 51.
Front glass substrate 50 and back glass substrate 51 are separately prepared, and finally sealed together with sealant 61 so as to form the sealed discharge space therebetween. The structure of the PDP is thus completed.
Referring to FIGS. 20A, 20B and 21, hereinafter is described a prior art method of manufacturing the PDP, including a step to form the discharge space shielded from the external space (i.e., the surrounding exterior space) with the above described sealant 61. FIGS. 20A and 20B illustrate a cross-sectional view and a plan view, respectively, of a PDP in a step for peripheral edge sealing; and FIG. 21 illustrates heating and exhausting processing cycles as a function of time.
Sealant 61 shown in FIGS. 20A and 20B has been formed by coating a glass paste on the back glass substrate 51 and, next, solidifying the paste during preparing of the back glass substrate. The thus prepared sealant is melted once during the sealing step and solidified again so as to join front glass substrate 51.
As shown in FIG. 20B, during the prior art process of sealing a PDP 71, a front glass substrate 73 and a back glass substrate 72 are stacked with a layer of sealant 74 between their respective peripheries and are clamped with several clips 77 at the peripheries thereof. Clips 77 both fix the glass substrates 72 and 73 relatively to each other as well as impose a predetermined pressure onto the peripheral portions to be sealed while the sealant 74 is melted.
That is, in order to form the discharge space 76 during the sealing process using sealant 74, it is necessary to melt the sealant 74 placed between the paired glass substrates 72 and 73 by heating same and to deform, i.e. press, the paired glass substrates 72 and 73 together so as to have the gap therebetween defined by the height of the separator walls. Accordingly, a pressure has to be imposed in a direction such that the paired glass substrates 72 and 73 approach each other. Several clips 77 are needed to generate the required pressure.
At the periphery of the back glass substrate 73, a conduction pipe (a glass pipe) 75 is provided so as to make a channel connecting the discharge space 76 and the outside (i.e., the exterior) of the PDP 71. The space 76 is exhausted of ambient air and then filled with a discharge gas via the pipe 75. During the prior art sealing process, a pair of the substrates 72 and 73, each of about 3 mm thickness, may be damaged by a stress due to direct clamping with many clips 77. Accordingly, it is necessary to seal the pair of substrates 72 and 73 while weakly clamped over a long time process.
The illustrative prior art method is explained in more detail with reference to FIG. 21, showing processing cycles in above described prior art. The pair of substrates 72 and 73, clamped with many clips 77 as shown in FIG. 20B, is carried into a furnace (not shown) for heating and then the seal head 5 (not shown) is closely mounted to the pipe 75. The seal head is connected to a pump for exhausting, and then to gas cylinders for gas filling (not shown in FIG. 20A).
While keeping (i.e., maintaining) such a state, a heater for heating the furnace is operated first so that the temperature inside the furnace is gradually raised so as to reach a melting temperature Tm of the sealant 74. This heating period is illustrated as a temperature-raising period T1. Next, the temperature inside the furnace is kept at the melting temperature Tm of sealant 74 for a predetermined period, which is illustrated as a first temperature-holding period T2. During the temperature holding period T2, sealant 74 is melted so as to allow both the front and back glass substrates to reach a predetermined gap therebetween defined by the height of the separator walls (e.g., as shown at 58 in FIG. 19) by the pressure of clips 77 as shown in FIGS. 20A and 20B.
The first temperature holding period T2 is a relatively long period because the process, during the temperature holding period T2, has to be carried out while the substrates 72 and 73 are clamped with clips having weak, or low, pressure as described above. When the gap between front glass substrate 72 and back glass substrate 73 reaches the predetermined gap size defined by the height of the separator walls, the temperature inside the furnace is decreased down to a solidifying temperature of sealant 74. This period is illustrated as a temperature-lowering period T3. During these periods of to T3, neither exhausting nor gas-filling is carried out from/into a discharge space 76 sealed by the sealing process.
Next, the temperature as lowered during the temperature lowering period T3 is held for a predetermined period, namely, a second, temperature holding period T4. This lowered temperature nevertheless is at a relatively high level, but such that sealant 74 does not melt. Upon beginning the second temperature-holding period T4, discharge space 76 is exhausted via an exhausting tube 75. This exhausting process is carried out in order to remove impurities existing in discharge space 76; accordingly, the temperature is kept at the high temperature T4 of second temperature holding period T4 sufficiently high as to drive out impurity gases adsorbed by the dielectric layers and the protection layers. The second temperature-holding period T4 is chosen according to the period required to complete removal of the impurity gases from the discharge space 76.
Next, the temperature inside the furnace is lowered by terminating the heater, as illustrated by a second temperature lowering period T5, during which the exhausting operation is continued so as to further remove the impurities. Upon completion of the impurity removal from the discharge space 76 and stabilization of the temperature inside the furnace at room temperature, illustrated as a room temperature period interval T6, a discharge gas is introduced, instead of the exhausting, via the conduction pipe 75 by switching a valve (not shown) provided on a pipe connected to the conduction pipe. The discharge gas is typically a mixture of neon gas and xenon gas.
By completing the processing cycle described above, the front glass substrate 72 and the back glass substrate 73 are sealed together by the sealant so as to form the discharge space 76 between these substrates 72 and 73.
In the above described prior art method, there is a possibility of breaking glass substrates 72 and 73 due to the stress caused from the many clips 77 directly contacting glass substrates 72 and 73. Therefore, the sealing process is carried out over a relatively long period with a weak dipping pressure.
Accordingly, a long period is required for the first temperature-holding period T2, that is a sealing process, resulting in the lowering of the process efficiency. Non-uniformity of the clip pressure may cause a local stress or cause an insufficiently pressed portion, whereby the glass substrate may be broken or may be incompletely sealed. The impurity removal from the discharge space, via the conduction pipe 75 only, also may cause a long exhausting period and insufficient purity in the discharge space.
It is a general object of the invention to provide a method of manufacturing a plasma display panel comprising a pair of substrates separated by a gas discharge space, which method is suitable for high efficient mass production and includes a process for reliably sealing of, and removal of impurities from, the gas discharge space.
The present invention provides a method of manufacturing a plasma display panel based on a feature that sealing a periphery of the pair of substrates is carried out with use of a force caused by a pressure difference between an interior of and an exterior of the pair of substrates during melting of the sealant. More specifically, the present invention provides a method of manufacturing a plasma display panel which comprises, sequentially, a first step of forming the sealant in a frame-shape on a periphery of at least one of the substrates and stacking one of substrates onto the other via the sealant, a second step of lowering the pressure in the space, closed with the sealant, between the stacked pair of substrates and of heating the sealant for melting same as so as to compress the sealant and define a gap between the substrates, a third step of curing the sealant, once melted, to glue and fix firmly the pair of substrates to each other and form a discharge space between the pair of the substrates, and a fourth step of removing impurities out of the discharge space.
In the method according to the present invention described above, the pair is pressed toward each other, pressing the sealant by the force due to the pressure difference between the outside and the inside of the pair, during melting of the sealant by heating. Accordingly, the external force applied to the pair may be minimized, a local stress caused in the prior art is decreased and the period for sealing the pair may be shortened, by the method of the present invention. The present invention is also desirable for high efficient mass-production of the panels owing to applying the method to a sealing process in the production process where a plurality of plasma display panels is cut out from a single pair of large substrates.
Further, the present invention provides a manufacturing method based on a feature that the gap of the discharge space in the three-electrodes surface discharge type PDP described above is maintained by a plurality of separator walls or ribs separating the discharge space and formed in a predetermined pattern on the inner surface of substrate. The method for sealing along the periphery of the pair of substrates at an interval, or distance, therebetween determined by the height of the walls, includes a step of forming, previously, on one of substrates a sealant in a frame-shape higher than that of the walls and of setting an assembly of the pair of substrates in a furnace able to heat and exhaust therein, and of exhausting the outside of the pair and in turn as well the inside during melting of the sealant.
Owing to the above described invention, the present invention may improve the dynamic and/or display characteristics, because exhausting the residual solid and/or gaseous impurities in the discharge space via a leakgap at a contact-portion of the sealant and the substrate is available in a period until the beginning of the sealant melting.
The invention described above improves color purity of light emitted from fluorescent material, which is formed on one of the pair, particularly on the back substrate, as well as the separator walls in the plasma display panels subject to the present invention, because heating to melt the sealant is carried out in forming a vacuum and also sufficient purification due to the use of pressure difference between in and outside of the pair is completed. On the other hand, the luminous characteristics, such as a color temperature, in plasma display panels produced via a prior art manufacturing method is poor due to a damage caused in a process in the method.
The above-mentioned features and advantages of the present invention, together with other objects and advantages, which will become apparent, will be more fully described hereinafter, with references being made to the accompanying drawings which form a part hereof, wherein like numerals refer to like parts throughout.